Add error message for including nokogiri in Gemfile for rake doc:guides [ci skip]

- When `rake doc:guides` is run from applications, it complains for
  presence of redcarpet if it not present in Gemfile
- Similarly it should complain about nokogiri
Merge pull request #11973 from prathamesh-sonpatki/nokogiri-warning

Add error message for including nokogiri in Gemfile for rake doc:guides